Starting from basic observations concerning commensurate structures, a simple explanation is given for the existence of energetically stable states of an fcc(111) adsorbate on a bcc(110) substrate, as a function of the mismatch and relative orientation of the two lattices. Numerical simulations are used to show how epitaxially rotated phases are favoured due to finite-size effects, and how adsorbate relaxation stabilizes higher-order commensurate structures.